A COMPARISON OF LOW OXYGEN EFFECTS IN THE PATUXENT RIVER AND MAINSTEM CHESAPEAKE BAY: IMPORTANCE OF THE SEVERITY AND STABILITY OF OXYGEN DEPLETION

The severity of oxygen depletion and temporal variability of oxygen concentrations may strongly influence the net effect of hypoxia and anoxia in coastal systems. Oxygen depletion in the mesohaline Chesapeake Bay is more severe, more stable and longer in duration than in its tributaries. Nevertheless, a comparison of the Patuxent River (a tributary of Chesapeake Bay) and mesohaline Bay indicated that bottom-layer hypoxia may cause greater mortality of fish eggs and more strongly alter the planktonic food web in some tributaries. Fish eggs were more abundant in the hypoxic bottom layer of the Patuxent, probably because of the weaker density stratification. Moderately hypoxic bottom waters in the Patuxent also resulted in stronger vertical separation of species as fish and invertebrates responded behaviorally to their species-specific physiological tolerances and to the behavior of their predators and prey. In contrast, severe oxygen depletion in the mesohaline mainstem concentrated virtually all animals into the surface and narrow pycnocline layers, resulting in less relative change in encounter rates among ichthyoplankton, zooplankton, and gelatinous predators, and few encounters at oxygen concentrations that alter predation rates.